The objective of this project is to expand understanding of health risks from polluted ambient air, by identifying specific pollutants and meteorological conditions which cause specific physiological and clinical effects. The program's aim is to address two main questions: (1) what are the effects of particle size on the biomedical response to acid fog or acid-polluted "dry" air? (2) which subgroups of the general population are most susceptible to respiratory disturbances from acid fog? To provide data to address these two issues, 24 healthy and 24 asthmatic volunteers will be exposed during the two program years in a controlled-environment chamber to worst-case simulated-ambient atmospheres containing sulfuric acid aerosol under two relative humidity conditions of interest [100+% (fog) and 85% ("dry")] and four particle sizes of potential biomedical concern [large and small fog (20 and 10 microns volume median diameter, or VMD, respectively), and large and small acid aerosol (1.0 and 0.5 microns, respectively)]. Measures of response will include whole body plethysmography, maximal forced expiratory performance, symptomatology, a cough-provocation test, and a post- exposure bronchial reactivity challenge (to methacholine chloride aerosol). Upon completion of this two year project we tentatively plan to propose two additional years of effort to study nitric acid, or a combination of nitric and sulfuric acids, in a similar manner, and pilot studies employing potentially at-risk population subgroups (i.e. chronic obstructive lung disease patients) will also be included.